Fresnel lenses have hitherto been produced by pressing, casting or the like. These methods, however, are disadvantageous in that a long time is required for lens preparation and, thus, the productivity is poor. In recent years, studies have been made on the preparation of lenses using ultraviolet-curable resin. Specifically, a lens can be produced in a short time by coating an ultraviolet-curable resin composition onto a lens-shaped mold, placing a transparent resin substrate on the resin composition coating to sandwich the resin composition coating between the transparent resin substrate and the mold, and applying ultraviolet light from the substrate side to cure the resin composition. A recent trend of projection televisions toward a reduction in thickness and an increase in size has led to the requirement that resins for lens formation have various lens characteristics such as increased refractive index and dynamic characteristics. Further, various proposals and studies have been made according to lens service environment.
A Fresnel lens for a projection screen has a construction provided by forming a lens shape using an ionizing radiation curing resin on a plastic substrate. The substrate is preferably formed of a material which is not optically anisotropic and is highly transparent. For example, polymethyl metacrylate (PMMA), a copolymer of metyl metacrylate with styrene (MS), polycarbonate (PC), and transparent olefin resins have hitherto been used as the substrate.
Further, in order to impart impact resistance to the substrate on which the shape of a lens (a cured product of an ionizing radiation-curable resin) is formed, a transparent material having a islands-sea structure formed by blending the above resin with butadiene rubber, acrylic rubber or the like is also often used.
In these plastic substrates, the selection of a proper substrate depending upon applications, properties and cost is very important. Improving the adhesion of the ultraviolet curing resin to the substrate by properly selecting the substrate is also of great significance from the practical point of view. For example, in order to improve the adhesion between the substrate and the ultraviolet curing resin, the substrate may be subjected to surface treatment (such as primer treatment, corona discharge treatment, flame treatment, ultraviolet treatment, or plasma treatment). The surface treatment effect varies depending upon the material. Further, the process and equipment for the surface treatment are also necessary. Therefore, from the viewpoint of production cost, that the lens material per se has good adhesion to the substrate is very effective. To this end, selecting a resin composition having good adhesion to the selected substrate is important from the viewpoint of shortening of a lead time on a mass production level.
In this connection, however, it should be noted that mere excellent adhesion of the ultraviolet curing resin to the substrate does not mean that properties required of the lens could have been satisfied. For example, the Fresnel lens is required to, of course, have optical properties such as refractive index and, in addition, to have vibration-damping properties, which suppress contact friction against the lenticular lens and rigidity or restorability, which can withstand the pressure of contact with the lenticular lens as disclosed in Japanese Patent Laid-Open No. 228549/2001. That is, the projection screen generally comprises two lens sheets different from each other in rigidity, i.e., a Fresnel lens and a lenticular lens. Vibration during the transportation of this screen causes friction between the two lenses, resulting in the occurrence of scratches on the surface of the lenses. In order to avoid this phenomenon, various measures have been taken such as regulation of the modulus of elasticity of the resin or interposition of a slip agent or a slip sheet. Further, silicone has been incorporated in the Fresnel lens resin to improve the friction resistance.
Lenses, formed by the photocurable resin composition, for use in projection televisions and the like are required to have properties, for example, optical properties such as high refractive index, excellent restorability and scratch resistance, and good adhesion between the substrate and the cured layer formed from the photocurable resin composition.
On the other hand, in the preparation of lenses, since a resin cured product should be released from the mold, the releasability of the resin cured product from the mold should be high.
At the present time, however, the resin compositions cannot simultaneously satisfy all the above property requirements, and provision of better resin compositions has been desired.
Accordingly, an object of the present invention is to provide a resin composition, which has optical properties such as a high refractive index, has excellent restorability and scratch resistance, and has good adhesion to a substrate, and, when utilized as an optical member (lens) used, for example, in screens for projection televisions or the like, can satisfy various property requirements, and to provide a cured product of the resin composition.
Another object of the present invention is to provide a resin composition which, even when molded into an optical member by a 2P method, does not deteriorate quality characteristics of the optical member and enables the optical member to be easily released from the mold.